This invention relates to a fuel burning method in a heating furnace and more particularly to a fuel burning method in a heating furnace for reducing the concentration of nitrogen oxides contained in the combustion gas exhausted from the furnace.
Various methods have heretofore been proposed for reducing the concentration of nitrogen oxides (NO.sub.x) contained in exhausted combustion gases. These methods are broadly classified into a method wherein an exhausted combustion gas is treated to reduce the concentration of NOx contained therein, and a method wherein the production itself of NOx is suppressed by improving fuel burning means. The former method is further classified into (a) a method not using a catalyst and (b) a method using a catalyst, while the latter method is further classified into (c) a method for improving burning conditions themselves on the basis of burner types, oxygen introduction, exhaust gas recycling, etc. and (d) a method wherein ancillary materials are fed at the time of combustion. This invention concerns an improved fuel burning method as an improvement from the method (d) just mentioned wherein ancillary materials are fed to suppress the production of NOx.
Typical of the above method (a) is a method (see Japanese Patent Publication No. 35908/75) wherein an ammonium compound is mixed with an exhaust gas at a high temperature ranging from 1600.degree. to 2000.degree. F. in the presence of a sufficient amount of oxygen. But this method is disadvantageous in that a fairly large amount of ammonia remains in the treated exhaust gas and in that if the temperature of the exhaust gas is low, it must be heated to raise its temperature. As to the above method (b), there have been proposed a very large number of techniques, but none of them have proved to be satisfactory in the durability and heat resistance of the catalysts used. The wet system also involves a problem in point of durability of a catalyst solution used therein. The above method (c) does not use an ancillary material and as the case may be it permits the use of an existing burning apparatus as it is, but the effect of reduction in the amount of NOx is not considered satisfactory. As to the above method (d), there have heretofore been known a method wherein water and alcohol are added and a method wherein a fuel is made into an emulsion by adding a surface active agent as the case may be. But a satisfactory result is not obtainable, either, and particularly in the case of an emulsified fuel, its preparation and storage are troublesome. There have also been proposed the addition of various metal salts (see, for example, Japanese Patent Laying Open Print No. 117001/78) and the addition of sodium thiosulfate (see Japanese Patent Publication No. 35400/80), but a drawback is unavoidable because metals and sulfur are contained in the exhaust gas.